Polyaxial bone screw with uploaded threaded shank and method of assembly and use

ABSTRACT

A polyaxial bone screw assembly includes a threaded shank body integral with an upper capture structure and a head having an inner cavity for receiving the capture structure. The capture structure is threaded and the head includes a threaded opening for rotatable assembly with the capture structure and eventual locking frictional engagement between the capture structure and the head. The head has a U-shaped cradle defining a channel for receiving a spinal fixation rod. The head channel communicates with the cavity and further with the threaded opening that allows for loading the capture structure into the head but prevents pushing or pulling of the capture structure out of the head. The capture structure and the head provide a ball joint, enabling the head to be disposed at an angle relative to the shank body. The capture structure includes a tool engagement formation and gripping surfaces for non-slip engagement by a tool for driving the shank body into bone.

BACKGROUND OF THE INVENTION

The present invention relates to apparatuses and methods for use inperforming spinal surgery and, in particular, to polyaxial bone screwsfor use in spinal surgery. Such screws have a head that can swivel abouta shank of the bone screw, allowing the head to be positioned in any ofa number of angular configurations relative to the shank.

Bone screws are utilized in many types of spinal surgery in order tosecure various implants to vertebrae along the spinal column. Spinalimplant screws typically have a shank that is threaded and configuredfor implantation into a pedicle or vertebral body of a vertebra. Such ascrew also includes a head designed to extend beyond the vertebra andalso defines a channel to receive a rod or other implant. In bone screwsof this type, the head may be open, in which case a closure member mustbe used to close between opposite sides of the head once a rod or otherimplant is placed therein. Alternatively, the head may be closed,wherein a rod-like implant is threaded through the head of the bonescrew.

When the head and shank of the bone screw are fixed in position relativeto each other, it is not always possible to insert a bone screw in sucha manner that the head will be in the best position for receiving otherimplants. Consequently, swivel head bone screws have been designed thatallow the head of the bone screw to rotate or swivel about an upper endof the shank of the bone screw while the surgeon is positioning otherimplants and finding the best position for the bone screw head. However,once the surgeon has determined that the head is in the best position,it is then necessary to lock or fix the head relative to the shank.Different types of structures have been previously developed for suchpurpose.

Because bone screws are for placement within the human body, it isdesirable for the implant to have as little effect on the body aspossible. Consequently, heavy, high profile, bulky implants areundesirable and lighter implants with a relatively small profile both inheight and width are more desirable. However, a drawback to smaller,lighter implants is that they may be more difficult to rigidly fix inposition relative to each other and in a desired position. Reduced bulkmay also reduce strength, resulting in slippage under high loading.Also, more component parts may be required to rigidly fix the implant ina desired position. A further drawback of smaller components is thatthey may be difficult to handle during surgery because of their smallsize, failing to provide adequate driving or gripping surfaces for toolsused to drive the shank into bone.

One undesirable attribute of some of the swivel-head implants is theneed for a multitude of components that may loosen or even disassemblewithin the body. It is most often undesirable for components to becomemoveable in the body after the completion of surgery. Loosening ofcomponents relative to each other may result in related undesirablemovement of the bone or vertebra that the implant was intended tostabilize.

SUMMARY OF THE INVENTION

A polyaxial bone screw assembly according to the present inventionincludes an elongate shank having a lower threaded body for fixation toa bone. The shank further has an upper capture structure connected tothe threaded body by a neck. The capture structure has an upper end ortop and a lower surface. The capture structure has a radially outwardsurface with a first helically wound guide and advancement structurethereon. The helically wound guide and advancement structure has a majordiameter (passing through the crests) that is larger than a diameter ofthe aperture into which the capture structure is inserted.

The assembly also includes a head having a base with an inner surfacedefining a cavity. The cavity opens onto a bottom of the head through aneck opening or aperture with a portion of the inner surface definingthe opening having a second helically wound guide and advancementstructure sized and shaped to rotatingly mate with the first guide andadvancement structure of the capture structure.

The capture structure screws upwardly into the head so as to be disposedwithin the cavity and captured by the head upon mating and operablerotation of the first guide and advancement structure with respect tothe second guide and advancement structure until the first guide andadvancement structure fully enters the cavity and becomes disengagedfrom the second guide and advancement structure. The capture structureis then disposed in the head cavity and free to rotate or swivelrelative to the head.

The capture structure has a first orientation wherein the capturestructure is within the cavity and the shank body is freely swivelablerelative to the head. In a second orientation, the capture structure isin a non-mated, frictional engagement with the second guide andadvancement structure and the shank body is in a fixed position withrespect to the head, resulting from a force being applied to the top ofthe capture structure.

Preferably according to the invention, the first and second guide andadvancement structures are first and second threads. Most preferablyaccording to the invention, inverted buttress threads are utilized,although square threads, reverse angle threads and other similar matingstructures can be utilized.

Also according to the invention, the elongate shank has a first axis andthe head has a second axis. The first and second guide and advancementstructures are configured to enter into frictional engagement when thecapture structure is urged downwardly against the base neck withoutrotation. The capture structure may expand in response to downwardpressure, further frictionally engaging and locking the first and secondguide and advancement structures.

Further according to the invention, a polyaxial bone screw capturestructure may include a tool engagement formation disposed at or nearthe top of the capture structure. Preferably, the tool engagementformation has a projection and a recessed tool seating surface with abottom and an outer wall. Both the bottom and outer wall are sized andshaped to receive and frictionally engage with a driving tool, such as asocket tool, engaged with the tool engagement projection for driving theshank body into bone. However, other structure for driving the shankbody can be used, such as off axis apertures into the threadedhemisphere.

A method according to the invention includes the steps of attaching abone screw shank to a head by mating a first helically wound guide andadvancement structure disposed on an upper portion of the bone screwshank with a second helically wound guide and advancement structure ofthe head to guide and advance the shank into the head until the firstguide and advancement structure becomes disengaged from the second guideand advancement structure with the capture structure or upper portionslidingly received and captured in a cavity of the head. Another methodstep includes driving the shank body into bone by rotating the shankbody with a tool engaged with a tool engagement formation disposed on orin the capture structure. The step of driving the shank into bone maytake place after or before the step of mating the bone shank with thehead.

OBJECTS AND ADVANTAGES OF THE INVENTION

Therefore, it is an object of the present invention to overcome one ormore of the problems with polyaxial bone screw assemblies describedabove. An object of the invention is to provide a shank that rotatablyuploads into a cavity in a head of the screw and that utilizesfrictional contact of threads under pressure to fix the head relative tothe shank once a desired configuration is acquired. Another object ofthe invention is to provide a polyaxial bone screw with features thatpresent frictional or gripping surfaces, planar surfaces, internalapertures or the like for bone implantation tools and may be readily andsecurely fastened to each other as well as to the bone. Also, if part ofthe implant should slip relative to another part or become loose forsome reason, an object of the invention is to provide an implant whereinall of the parts remain together and do not separate. Furthermore, it isan object of the invention to provide a lightweight, reduced volume, lowprofile polyaxial bone screw that assembles in such a manner that thecomponents cooperate to create an overall structure that preventsunintentional disassembly. Furthermore, it is an object of the inventionto provide apparatus and methods that are easy to use and especiallyadapted for the intended use thereof and wherein the tools arecomparatively inexpensive to produce.

Other objects and advantages of this invention will become apparent fromthe following description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of this invention.

The drawings constitute a part of this specification and includeexemplary embodiments of the present invention and illustrate variousobjects and features thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a polyaxial bone screwassembly according to the present invention having a shank with acapture structure at an end thereof, a head, and a closure structure.

FIG. 2 is an enlarged and fragmentary view of the assembly of FIG. 1,showing the head in cross-section, taken along the line 2-2 of FIG. 1,and illustrating the shank in front elevation prior to the insertion ofthe shank capture structure into the head according to a method of theinvention.

FIG. 3 is a reduced and fragmentary cross-sectional view of the head andshank, taken along the line 2-2 of FIG. 1, showing the shank capturestructure partially screwed into the head.

FIG. 4 is a reduced and fragmentary cross-sectional view of the head andshank of FIG. 3, illustrating the shank capture structure disposed androtatable within the head.

FIG. 5 is a reduced and fragmentary cross-sectional view of the head andthe attached shank of FIG. 4, and further showing the shank beingimplanted into a vertebra using a driving tool mounted on the shankcapture structure.

FIG. 6 is an enlarged and fragmentary cross-sectional view of the head,shank and driving tool of FIG. 5.

FIG. 7 is a reduced and fragmentary cross-sectional view of the head,similar to FIG. 5, showing the shank in front elevation and implanted inthe vertebra, a rod, in cross-section, disposed in the head, andillustrating the insertion of the closure structure using a driver.

FIG. 8 is a reduced front-elevational view of the assembly of FIG. 1,shown with a rod in cross-section, the shank implanted in the vertebraand with the closure structure fully installed.

FIG. 9 is an enlarged and fragmentary view of the assembly of FIG. 8with the head and rod in cross-section, showing the details thereof.

FIG. 10 is a front-elevational view of the shank of FIG. 1 shownimplanted in a vertebra (shown in cross-section) according to analternative method of the invention.

FIG. 11 is a front-elevation view of the shank of FIG. 10, and includinga reduced cross-sectional view of the head of FIG. 2, illustratinginsertion of the head on the implanted shank according to an alternativemethod of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention, which may be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure.

With reference to FIGS. 1-9, the reference number 1 generally representsa polyaxial bone screw apparatus or assembly according to the presentinvention. The assembly 1 includes a shank 4 and a head 6. The shank 4further includes a body 8 integral with an upwardly extending capturestructure 10. The shank 4 and head 6 are often assembled prior toimplantation of the shank body 8 into a vertebra 13, as seen in FIGS. 3and 4. However, in a method of the invention shown in FIGS. 10 and 11,the shank body 8 is first implanted in the vertebra 13, followed byjoining the head 6 to the shank 4.

FIG. 1 further shows a closure structure 16 of the invention for biasinga longitudinal member such as a rod 19 against the capture structure 10which in turn biases the structure 10 into fixed frictional contact withthe head 6, so as to fix the rod 19 relative to the vertebra 13. Thehead 6 and shank 4 cooperate in such a manner that the head 6 and shank4 can be secured at any of a plurality of angles, articulations orrotational alignments relative to one another and within a selectedrange of angles both from side to side and from front to rear, to enableflexible or articulated engagement of the head 6 with the shank 4 untilboth are locked or fixed relative to each other near an end of animplantation procedure.

With reference to FIGS. 1 and 2, the shank 4 is elongate, with the shankbody 8 having a helically wound bone engaging thread 24 extending fromnear a neck 26 located adjacent to the capture structure 10 to near atip 28 of the body 8 and projecting radially outward therefrom.

During use, rotation of the body 8 utilizes the thread 24 for grippingand advancement in the bone and is implanted into the vertebra 13leading with the tip 28 and driven down into the vertebra 13 with aninstallation or driving tool 31, so as to be implanted in the vertebra13 to near the neck 26, as shown in FIG. 5 and as is described morefully in the paragraphs below.

The shank 4 has an elongate axis of rotation generally identified by thereference letter A. It is noted that any reference to the words top,bottom, up and down, and the like, in this application refers to thealignment shown in the various drawings, as well as the normalconnotations applied to such devices, and is not intended to restrictpositioning of the assembly 1 in actual use.

The neck 26 extends axially outward and upward from the shank body 8 toa base 34 of the capture structure 10. The neck 26 generally has areduced radius as compared to an adjacent top 36 of the shank body 8.Further extending axially and outwardly from the neck 26 is the capturestructure 10 that provides a connective or capture apparatus disposed ata distance from the body top 36 and thus at a distance from the vertebra13 when the shank body 8 is implanted in the vertebra 13.

The capture structure 10 is configured for connecting the shank 4 to thehead 6 and then capturing the shank 4 in the head 6. The capturestructure 10 has an outer substantially hemi-spherically or partialspherically shaped surface 40 extending from the base 34 to a topportion 44. Formed on an upper part 46 of the surface 40 is a guide andadvancement structure illustrated in the drawing figures as an invertedor reverse buttress thread 48. The thread 48 is sized and shaped to matewith a cooperating guide and advancement structure 50 disposed on aninner surface 52 of the head 6 disposed adjacent to and defining anopening 54 of a lower end or bottom 56 of the head 6. Preferably, thethread 48 is relatively thick and heavy to give strength to the threadand prevent the threads from being easily bent or deformed when axialpressure is applied to the shank 4 to maintain the capture structure 10in the head 6, as described further below.

The thread 48 winds about the upper portion 46 in a generally helicalpattern or configuration that is typical of threads and can have variouspitches, be clockwise or counterclockwise advanced, or vary in most ofthe ways that conventional buttress threads vary. The thread 48 has aleading surface or flank 58 and a trailing surface or flank 59. As usedherein, the terms leading and trailing refer to the direction ofadvancement of the capture structure 10 into the guide and advancementstructure 50 of the head 6 aligning the axis A of the shank 4 with anelongate axis of rotation B of the head 6 and directing the capturestructure 10 toward the head 6, as shown by the straight arrow Cillustrated in FIG. 2.

The leading surface 58 has an inner edge 62 and an outer edge 63. Thetrailing surface 59 has an inner edge 66 and an outer edge 67. At thecrests of the thread 48, where the leading surface outer edge 63 and thetrailing surface outer edge 67 meet or are closely spaced relative toone another, preferably there is a slight relief as shown in thedrawings so as to have a slight connecting wall or crest surface 70therebetween along a substantial length of the thread that decreases thesharpness of the buttress thread 48 and increases the strength andsurface contact thereof. The size of the crest or connecting surface 70varies, generally increasing as the thread 48 winds from the top surface44 to a non-threaded lower portion 72 of the surface 40.

As can be seen in the drawing figures, the general shape of the crosssection of the thread 48 is that of a right triangle, with the leadingsurface 58 sloping away from the axis A and downwardly from the inneredge 62 and the trailing surface 59 extending substantially horizontallyfrom the inner edge 66 and thus substantially perpendicular to the axisA.

Although a reverse or inverted buttress thread as described herein ispreferable for use according to the invention, it is foreseen that otherthread types, such as V-threads, square threads, other inverted threadtypes or other thread like or non-thread like guide and advancementstructures, such as flange form, helically wound advancement structuresmay be utilized according to the invention. Other preferred thread-typesalso include square threads with wide strong teeth and greater surfacecontact as well as modified inverted buttress threads, for examplebuttress threads wherein the angular relationship between the trailingand leading surfaces are modified somewhat, or wherein the size, shapeor orientation of the connecting wall between the leading and trailingsurfaces is modified somewhat.

Advancement of the capture structure 10 into the head 6 is accomplishedby rotating the shank 4 in a counterclockwise direction about the axes Aand B and into the head 6 as illustrated by the arrow T in FIG. 2. Aswill be described more fully below, an outer edge of the trailingsurface or flank 59 and/or the connecting surface 70 may also be aloading surface after the capture structure 10 is fully disposed in thehead 6.

The non-threaded lower portion 72 of the capture structure 10 surface 40that is disposed between the base 34 and the thread 48 may have a smoothor a high-friction or roughened surface, such as a scored or knurledsurface 73 illustrated on FIG. 9. As also illustrated in FIG. 9 and willbe described more fully below, the lower portion 72 may come intocontact with the head guide and advancement structure 50 during a rodreduction process according to the present invention.

In the embodiment shown, the shank 4 further includes a rod and toolengagement structure 74 projecting upwardly from the top portion 44 ofthe capture structure 10. The tool engagement structure 74 has ahexagonally shaped head 76 with a substantially domed top. 78. Thestructure 74 is coaxial with both the threaded shank body 8 and thecapture structure 10. The head 76 is sized and shaped for engagementwith the driving tool 31 shown in FIGS. 5 and 6 that includes a drivingand mating structure in the form of a socket. The tool 31 is configuredto fit about the head 76 so as to form a socket and mating projectionfor both operably driving and rotating the shank body 8 into thevertebra 13.

In the embodiment shown, to provide further mechanical advantage duringinstallation of the shank 4 into the vertebra 13, the capture structure10 includes a counter-sunk portion 80 formed in the top 44, the portion80 adjacent to and surrounding the head 76. The portion 80 includes aplanar seating surface 82 disposed perpendicular to the axis A andspaced from the top portion 44. Contiguous to both the surface 82 andthe top 44 are faces 84 that are disposed parallel to the axis A andthus are substantially perpendicular to the surface 82. The faces 84form a hex-shaped outer periphery of the counter-sunk portion 80. Thetool 31 includes an outer surface portion 90 sized and shaped to matewith the bottom and both side walls of the counter-sunk portion 80, suchthat a bottom of the tool 31 seats on the surface 82 and the outersurface portion 90 is adjacent to and engaging the faces 84 when thetool 31 is disposed about and engaging with the hexagonally shaped head76.

The domed top end surface 78 of the shank 4 is preferably convex, curvedor dome-shaped as shown in the drawings, for positive engagement withthe rod 19 when the bone screw assembly 1 is assembled, as shown inFIGS. 7-9, and in any alignment of the shank 4 relative to the head 6.In certain embodiments, the surface 78 is smooth. While not required inaccordance with the practice of the invention, the surface 78 may bescored or knurled to further increase frictional engagement between thedome 78 and the rod 19. The dome 78 may be radiused so that the dome 78engages the rod 19 slightly above a surface 100 defining a lower portionof a rod receiving channel in the head 6, even as the head 6 isswivelled relative to the shank 4 so that pressure is always exerted onthe dome surface 78 by the rod 19 when the assembly 1 is fullyassembled. It is foreseen that in other embodiments the dome 78 can haveother shapes which may include off-axis apertures for driving the shankwith a mating tool.

The shank 4 shown in the drawings is cannulated, having a small centralbore 92 extending an entire length of the shank 4 along the axis A. Thebore 92 is defined by an inner substantially cylindrical wall 95 of theshank 4 and has a first circular opening 96 at the shank tip 28 and asecond circular opening 98 at the top domed surface 78. The bore 92 iscoaxial with the threaded body 8 and the capture structure 10. The bore92 provides a passage through the shank 4 interior for a guide pin orlength of wire 103 inserted into a small pre-drilled tap bore 105 in thevertebra 13 prior to the insertion of the shank body 8, the pin 103providing a guide for insertion of the shank body 8 into the vertebra13.

The head 6 is partially cylindrical in external profile and includes abase portion 110 extending from the end 56 to a V-shaped surface 111disposed at a periphery of the surface 100 and extending radiallyoutwardly and downwardly therefrom. The base 110 is integral with a pairof upstanding and spaced arms 112 and 114. The surface 100 and the arms112 and 114 forming a U-shaped channel 116 between the arms 112 and 114with an upper opening 119. The lower surface 100 defining the channel116 preferably has substantially the same radius as the rod 19. Inoperation, the rod 19 preferably is located just above the channel lowersurface 100, as shown in FIGS. 7-9.

Each of the arms 112 and 114 has an interior surface 122 that defines aninner cylindrical profile and includes a discontinuous helically woundguide and advancement structure 124 beginning at a top 125 of the head 6and extending downwardly therefrom. The guide and advancement structure124 is a partial helically wound flange-form configured to mate underrotation about the axis B with a similar structure disposed on theclosure structure 16, as described more fully below. However, it isforeseen that the guide and advancement structure 124 couldalternatively be a V-shaped thread, a buttress thread, a reverse anglethread or other thread-like or non-thread-like helically wound guide andadvancement structure for operably guiding under rotation and advancingthe closure structure 16 between the arms 112 and 114, as well aseventual torquing when the closure structure 16 abuts against the rod19.

The head 6 includes external, grip bores 128 and 129 disposed on therespective arms 112 and 114 for positive engagement by a holding tool(not shown) to facilitate secure gripping of the head 6 during assemblyof the head 6 with the shank 4. Furthermore, the grip bores 128 and 129may be utilized to hold the head 6 during the implantation of the shankbody 8 into the vertebra 13. The bores 128 and 129 are centrally locatedon the respective arms 112 and 114 and may communicate with upwardlyprojecting hidden recesses to further aid in securely holding the head 6to a holding tool (not shown). It is foreseen that the bores 128 and 129may be configured to be of a variety of sizes, shapes and locationsalong outer surfaces of the arms 112 and 114.

Communicating with the U-shaped channel 116 of the head 6 is a chamberor cavity 136 substantially defined by a partially spherical innersurface 138 that is disposed in the base portion 110 of the head beneaththe interior cylindrical surface 122 of the arms 112 and 114 andextending into the inner surface 52 that defines the guide andadvancement structure 50. The cavity 136 communicates with both theU-shaped channel 116 and a bore 140 that also is defined by the guideand advancement structure 50, that in turn communicates with the opening54 at the bottom 56 of the head 6.

The guide and advancement structure 50 includes a leading surface 152and a trailing surface 156. Similar to what is described herein withrespect to the reverse buttress thread 48 of the capture structure 10,the guide and advancement structure 50 is preferably of a buttressthread type as such structure provides strength and stability to theassembly 1, with the trailing surface 156 that extends substantiallyperpendicular to the axis B. The cross-sectional configuration of aninverted buttress thread also results in an orientation for thestructure 50 that improves strength and desirably resists pushing of thecapture structure 10 out of the opening 54. However, as with the thread48, it is foreseen that other types of threaded and non-threaded helicalstructures may be utilized in accordance with the present invention.

A juncture of the interior surface 122 and the cavity inner surface 138forms an opening or neck 158 that has a radius extending from the Axis Bthat is smaller than a radius extending from the Axis B to the innersurface 138. Also, a radius from the lower opening 54 to the Axis B issmaller than the radius extending from the Axis B to the inner surface138 and the inner surface portion 52 defining the guide and advancementstructure 50. Thus, the cavity or chamber 136 is substantiallyspherical, widening and opening outwardly and then inwardly in adirection toward the lower opening 54. However, it is foreseen thatother shapes, such as a cone or conical shape, may be utilized for ahead inner cavity according to the invention.

After the reverse buttress thread 48 of the capture structure 10 ismated and rotated to a position within the cavity 136 and furtherupwardly and axially into non-engagement beyond the trailing surface 156of the guide and advancement structure 50, the capture structure 10 isrotatable or swingable within the cavity 136 until later frictionallylocked in place, and cannot be removed from the head 6 through the upperneck 158 or through the lower bore 140 without reversing the assemblyprocess with the components in axial alignment. As shown in FIG. 4, thecapture structure 10 is held within the cavity 136 from above by thepartially spherical surface 138 and from below by the threaded innersurface 52. Stated in another way, the thick strong thread 50 of thehead 6 disposed along the surface 52, and the unmated, thick strongthread 48 of the capture structure 10, prevent the capture structure 10from being pushed or pulled from the chamber 136, unless the capturestructure 10 is rotated and unscrewed therefrom again through the bore140 in axial alignment. More specifically, the buttress thread 48,particularly the trailing surface 59, resists pushing out of the bore140 and bottom opening 54 due to the strength and orientation of thebuttress thread and the fact that the greatest diameter of the threadedportion 46 of the capture structure 10 is greater than the interiordiameter of the bore 140.

As shown in FIG. 9 and described more fully below, the buttress thread48 and mating thread 50 further provide a frictional interface whenpushed from above, as by a closure structure 16 pushing on a rod 19 orother tool pushing against the dome 78, with outer edges of the thread48 contacting the inner surface 52 or portions of the thread 50,resulting in a digging or abrasion into the surface 52 by the thread 48.However, if there is no pushing from above, the cavity or chamber 136allows the structure 10 to freely rotate in the chamber 136 to aposition or orientation desired by a surgeon. In this manner, the head 6is able to swivel or swing about the shank 4 until subsequently lockedin place.

The elongate rod or longitudinal member 19 that is utilized with theassembly 1 can be any of a variety of implants utilized inreconstructive spinal surgery, but is normally a cylindrical elongatestructure having a cylindrical surface 162 of uniform diameter andpreferably having a generally smooth surface. The rod 19 is alsopreferably sized and shaped to snugly seat near the bottom of theU-shaped channel 116 of the head 6 and, during normal operation, ispositioned slightly above the bottom of the channel 116 near, but spacedfrom, the lower surface 100.

In particular, the rod 19 normally directly or abutingly engages theshank top surface 78, as shown in FIGS. 8 and 9 and is biased againstthe dome shank top surface 78, consequently biasing the shank 4downwardly in a direction toward the base 110 of the head 6 when theassembly 1 is fully assembled with the rod 19 and the closure member 16.For this to occur, the shank top surface 78 must extend at leastslightly into the space of the channel 116, above the surface 100 whenthe capture structure 10 is snugly seated in the lower part of the headcavity 136 as shown in FIG. 9 with a portion of the buttress thread 48contacting a portion of the structure 50, resulting in a frictionalinterface between the thread 48 and the thread 50. The pressure placedon the capture structure 10 by the rod 19 and closure member 16 may alsocause a spreading or expansion of the capture structure 10, causing aninterlocking or interdigitation of the threads 48 and 50, or an abradingof the surface 52 at the thread 50 by the thread 48. The shank 4 and thecapture structure 10 are thereby locked or held in position relative tothe head 6 by the rod 19 firmly pushing downward on the shank domedsurface 78.

With reference to FIGS. 1, 7 and 8, the closure structure or closure top16 can be any of a variety of different types of closure structures foruse in conjunctions with the present invention with suitable matingstructure on the upstanding arms 112 and 114. The closure top 16 screwsbetween the spaced arms 112 and 114 and closes the top of the channel116 to capture the rod 19 therein.

The illustrated closure top 16 has a generally cylindrically shaped body170, with a helically wound guide and advancement structure 172 that issized, shaped and positioned so as to engage the guide and advancementstructure 124 on the arms 112 and 114 to provide for rotatingadvancement of the closure structure 16 into the head 6 when rotatedclockwise and, in particular, to cover the top or upwardly open portionof the U-shaped channel 116 to capture the rod 19, preferably withoutsplaying of the arms 112 and 114. The body 170 further includes a baseor bottom 174 having a pointed rod engaging projection or point 175extending or projecting axially beyond a lower rim 176. The closurestructure 16, with the projection 175 frictionally engaging and abradingthe rod surface 162, thereby applies pressure to the rod 19 undertorquing, so that the rod 19 is urged downwardly against the shank domedsurface 78 that extends into the channel 116. Downward biasing of theshank surface 78 operably produces a frictional engagement between therod 19 and the surface 78 and also urges the capture structure 10 towardthe base 110 of the head 6, as will be described more fully below, so asto frictionally seat the capture structure buttress thread 48 and/orlower portion 72 against the threaded inner surface 52 of the head 6,also fixing the shank 4 and capture structure 10 in a selected, rigidposition relative to the head 6.

The illustrated closure structure 16 further includes a substantiallyplanar top surface 178 that has a centrally located, hexalobularinternal driving feature 180 formed therein (sold under the trademarkTORX),which is characterized by an aperture with a 6-point star-shapedpattern. It is foreseen that other driving features or apertures, suchas slotted, hex, tri-wing, spanner, and the like may also be utilizedaccording to the invention. With reference to FIG. 7, a driving/torquingtool 179 having a cooperating hexalobular driving head is used to rotateand torque the closure structure 16. The tool 179 may also be utilizedfor removal of the closure structure 16, if necessary.

It is foreseen that a closure structure according to the invention maybe equipped with a break-off feature or head, the closure structuresized and shaped to produce a break-way region that breaks at apreselected torque that is designed to properly seat the closurestructure in the head 6. Such a closure structure would include removaltool engagement structure, such as a pair of spaced bores, a countersunkhex-shaped aperture, a left hand threaded bore, or the like, fullyaccessible after the break-off head feature breaks away from a base ofthe closure structure.

In use, prior to the polyaxial bone screw assembly 1 being implanted ina vertebra according to the invention, the shank capture structure 10 isoften pre-loaded by insertion or bottom-loading into the head 6 throughthe opening 54 at the bottom end 56 of the head 6. The capture structure10 is aligned with the head 6, with the axes A and B aligned so that thereverse buttress thread 48 of the capture structure 10 is inserted intoand rotatingly mated with the guide and advancement structure 50 on thehead 6. The shank 4 is rotated in a counter-clockwise direction asillustrated by the arrow T in FIG. 2 to fully mate the structures 48 and50, as shown in FIG. 3, and the counter-clockwise rotation is continueduntil the thread 48 disengages from the thread 50 and the capturestructure 10 is fully disposed in the head cavity 136.

In the position shown in FIG. 4, the shank 4 is in slidable androtatable engagement with the head 6, while the capture structure 10 ismaintained in the head 6 with the shank body 8 in rotational relationwith the head 10. For example, an extent of rotation is shown in FIGS. 4and 9 where it can be deduced that the shank body 8 can be rotatedthrough a substantial angular rotation relative to the head 6, both fromside to side and from front to rear so as to substantially provide auniversal or ball joint wherein the angle of rotation is only restrictedby engagement of the thread 48 of the capture structure 10 and thethread 50 of the head 6 at a lower portion or area of the head 6 and bythe thread 48 contacting the inner spherical surface 138 of the head 6at an upper portion or area of the head 6.

With reference to FIGS. 5 and 6, the assembly 1 is then typicallyscrewed into a bone, such as the vertebra 13, by rotation of the shankbody 8 using the driving tool 31 that operably drives and rotates theshank 8 by engagement thereof with the hexagonally shaped extension head76 of the shank 4. Preferably, when the driving tool 31 engages the head76 during rotation of the driving tool 31, the outer portion 90 alsoengages the faces 84 and a bottom of the tool 31 is fully seated uponand frictionally engages with the planar surface 82 disposed in thecounter-sunk portion 80 of the capture structure 10. It is foreseen thatin other embodiments according to the invention, the counter-sunkportion may be defined by more or fewer engaging surfaces.

With particular reference to FIG. 5, the vertebra 13 may be pre-drilledwith the small tap bore 105 to minimize stressing the bone andthereafter have the guide wire or pin 103 inserted therein to provide aguide for the placement and angle of the shank 4 with respect to thevertebra 13. A further tap bore (not shown) may be made using a tap withthe guide pin 103 as a guide. Then, the assembly 1 is threaded onto theguide pin 103 utilizing the cannulation bore 92 by first threading thepin 103 into the bottom opening 96 and then out of the top opening 98.The shank body 8 is then driven into the vertebra 13, using the pin 103as a placement guide.

With reference to FIGS. 7-9, the rod 19 is eventually positioned withinthe head U-shaped channel 116, and the closure structure or top 16 isthen inserted into and advanced in a clock-wise direction between thearms 112 and 114 so as to bias or push against the rod 19. The closurestructure 16 is rotated, utilizing the tool 179 in engagement with thedriving feature or aperture 180 until an appropriate torque is achieved,for example 90 to 120 inch pounds, to urge the rod 19 downwardly. Theshank top domed surface 78, because it is rounded to approximatelyequally extend upward into the channel 116 approximately the same amountno matter what degree of rotation exists between the shank 8 and thehead 6 and because the surface 78 is sized to extend upwardly into theU-shaped channel 116, the surface 78 is engaged by the rod 19 and pusheddownwardly toward the base 110 of the head 6 when the closure structure16 biases downwardly toward and onto the rod 19.

In very unusual circumstances, the Axis A and the Axis B are aligned andin such a case the surface 72 of the capture structure 10 engages andsets atop the thread 50 of the head 6. Downward pressure on the shank 4produces frictional fixing between the surface 72 and the thread 50 insuch an alignment.

In most final placements, the head 6 is tilted relative to the shank 4,so that the Axes A and B are not aligned. In such a situation, downwardpressure on the shank 4 in turn urges the capture structure 10 downwardtoward the head inner surface 52 and associated guide and advancementstructure 50, with a portion of the buttress thread 48 being urged intofrictional engagement with a portion of the threaded surface 52 on thehead 6. Further, another portion of the thread 50 engages andfrictionally locks with a portion of the capture structure surface 72,as seen in FIG. 9. As the closure structure 16 presses against the rod19, the rod 19 presses against the shank 4, and the capture structure 10becomes frictionally and rigidly attached to the head 10. Outer edgesformed by the leading 58 and trailing 59 surfaces of the thread 48frictionally engage and abrade the inner threaded surface 52 and thespherical surface 138. If the pressure is such that the capturestructure 10 expands, a meshing and/or interlocking of the thread 48 andthe thread 50 may occur. Thus, this interlocking or meshing of thesurfaces of the thread 48 with the surfaces of the thread 50 furtherfixes the shank body 8 in a desired angular configuration with respectto the head 6 and the rod 19.

FIG. 8 illustrates the polyaxial bone screw assembly 1 with the rod 19and the closure structure 16 positioned in a vertebra 13. The axis A ofthe bone shank 8 is illustrated as not being coaxial with the axis B ofthe head 6 and the shank body 8 is fixed in this angular lockedconfiguration.

Other angular configurations can be achieved, as required duringinstallation surgery due to positioning of the rod 19 or the like. Withreference to FIG. 9, an implanted polyaxial bone screw assembly 1 isshown wherein the shank body 8 is fixed in a desired angular orientationwith respect to the head 6 with the rod 19 in frictional contact withthe domed surface 78, a portion of the wall 70 disposed between theleading surface 52 and the trailing surface 59 being in frictionalcontact with the thread 50 of the head 6, and a portion of the lowerspherical surface 72 of the capture structure 10 in contact with thethread 50 of the head 6.

It is foreseen that, when the shank 4 is not disposed at an angle withrespect to the head, in other words, when the axes A and B remainaligned and the shank body 8 is locked into a position substantiallycoaxial with the head, then the surface 72 abuts against the guide andadvance structure 50 only. Such a locked position adequately holds theshank in place, with outer edges of the thread 50 frictionally engagingand abrading the surface 72, but as noted before, this is not common.The shank 4 typically is locked into place with a portion of the thread48 frictionally interfacing with the thread 50. It is foreseen thataccording to the invention, the geometry of the surface 72 may bemodified slightly so that when a coaxial orientation of the shank 4 andthe head 6 is desired, the buttress thread 48 will frictionally engagewith the thread 50 with no contact being made between the head 6 and thecapture structure 10 at either the spherical surface 138 or thespherical surface 72.

If removal of the assembly 1 and associated rod 19 and closure structure16 is necessary, disassembly is accomplished by using the driving tool179 that is received in and mates with the driving feature 180 and thenturned counterclockwise to rotate the closure structure 16 and reversethe advancement thereof in the head 6. Then, disassembly of the assembly1 is continued in reverse order to the procedure described previouslyherein for assembly.

With reference to FIGS. 10 and 11, in an alternative method according tothe invention, the shank 4 is first implanted into the vertebra 13 byrotation of the shank 8 into the vertebra 13 using the driving tool 31that operably drives and rotates the shank 8 by engagement thereof withthe hexagonally shaped extension head 76 of the shank 4. As alreadydescribed herein, when the driving tool 31 engages the head 76 duringrotation of the driving tool 31, the outer portion 90 also engages thefaces 84 and a bottom of the tool 31 is fully seated upon andfrictionally engages with the planar surface 82 disposed in thecounter-sunk portion 80 of the capture structure 10. It may be desirableto only partially implant the shank 8 into the vertebra 13, with thecapture structure 10 extending proud to provide space for the attachmentof the head 6 to the shank 4.

The head 6 is then attached to the shank 4 by inserting the head 6 ontothe capture structure with the axes A and B aligned and mating thethread 48 with the thread 50 by rotating the head 6 in a clockwisedirection. The head is then rotated until the thread 48 disengages withthe thread 50 and the capture structure 10 is freely rotatably disposedin the head cavity 136. Then, the shank body the shank 4 can be furtherdriven into the vertebra 13, if necessary, utilizing the driving tool 31as already described herein. The remainder of the implant assemblyincludes elements that have been previously described.

It is to be understood that while certain forms of the present inventionhave been illustrated and described herein, it is not to be limited tothe specific forms or arrangement of parts described and shown.

1. A polyaxial bone screw assembly comprising: a) an elongate shankhaving a body for fixation to a bone and a capture structure having afirst helically wound guide and advancement structure with a majordiameter; and b) a head having a second helically wound guide andadvancement structure defining a capture structure loading aperturehaving a diameter, the first helically wound guide and advancementstructure mateable with the second helically wound guide and advancementstructure, the loading aperture communicating with a cavity sized andshaped to swivelabley receive the capture structure therein, the majordiameter of the first helically wound guide and advancement structurebeing larger than the loading aperture diameter.
 2. The assembly ofclaim 1 wherein the first helically wound guide and advancementstructure is an inverted thread.
 3. The assembly of claim 1 wherein thefirst helically wound guide and advancement structure is an invertedbuttress thread.
 4. The assembly of claim 1 wherein the capturestructure is substantially spherical.
 5. The assembly of claim 4 whereinthe second helically wound guide and advancement structure winds about asubstantially spherical surface.
 6. The assembly of claim 1 wherein thecapture structure has a tool engagement formation disposed thereonadapted for non-slip engagement by a tool for driving the shank bodyinto bone.
 7. The assembly of claim 6 wherein the capture structure toolengagement formation is an upward axial projection having a hexagonalprofile.
 8. A polyaxial bone screw assembly comprising: a) an elongateshank having a body for fixation to a bone and a capture structureextending from the body, the capture structure having a rod-engagementend and a first helically wound guide and advancement structure; and b)a head having a base with an inner surface defining a cavity, the innersurface having a second helically wound guide and advancement structuresized and shaped to rotatingly mate with the first guide and advancementstructure, the base cavity communicating with an exterior of the basethrough an opening located adjacent the second guide and advancementstructure, the capture structure being disposed and captured in thecavity upon operable mating and rotation of the first guide andadvancement structure with the second guide and advancement structureuntil the first guide and advancement structure becomes disengaged fromthe second guide and advancement structure, the capture structure havinga first orientation wherein the capture structure is disposed in thecavity and the shank body is swivelable relative to the head, and asecond orientation, wherein the capture structure is in a non-threadedlymated frictional engagement with the second guide and advancementstructure, the rod-engagement end is in frictional engagement with arod, and the shank body is in a fixed position with respect to the head.9. The assembly of claim 8 wherein the first and second guide andadvancement structures are first and second threads.
 10. The assembly ofclaim 9 wherein the first thread is an inverted buttress thread.
 11. Theassembly of claim 8 wherein a portion of the first guide and advancementstructure frictionally engages with a portion of the second guide andadvancement structure in said second orientation.
 12. The assembly ofclaim 8 wherein the capture structure has a tool engagement formationdisposed thereon adapted for non-slip engagement by a tool for drivingthe shank body into bone.
 13. The assembly of claim 12 wherein thecapture structure tool engagement formation is an upward axialprojection having a hexagonal profile.
 14. The assembly of claim 12wherein the tool engagement formation is a projection and the capturestructure has a tool seating surface, the projection and the toolseating surface partially defining a recess adapted for receiving adriving tool engaged with the tool engagement projection and wherein thedriving tool is adapted to be in contact with the tool seating surfacewhen driving the shank body into bone.
 15. The assembly of claim 14wherein the capture structure has at least one engagement wall disposedperpendicular to the tool seating surface and spaced from theprojection, the engagement wall partially defining the recess forreceiving the driving tool and wherein the driving tool is adapted to bein contact with the tool seating surface, the projection and theengagement wall when driving the shank body into bone.
 16. The assemblyof claim 8 wherein the shank is cannulated from the top to a bottomthereof.
 17. The assembly of claim 8 further comprising a closurestructure insertable into an upper opening of the head, the closurestructure for indirectly operably urging the shank in a direction tofrictionally lock the capture structure to the head in the secondorientation thereof, thereby locking the shank body in a selected anglewith respect to the head.
 18. The assembly of claim 17 wherein theclosure structure has a hexalobular internal driving feature.
 19. Theassembly of claim 17 wherein: (a) the head has upstanding spaced armsdefining an open channel, the arms having a third guide and advancementstructure on an inside surface thereof, the channel communicating withthe cavity; and (b) the closure structure is sized and shaped to bepositionable between the arms for closing the channel, the closurestructure having a fourth guide and advancement structure for rotatablymating with the third guide and advancement structure, so as to beadapted to bias the closure structure upon advancement rotation againsta rod disposed in the channel.
 20. The assembly of claim 19 wherein thecapture structure top has a dome sized and shaped to extend into thechannel so as to be adapted to engage a rod received in the head andwherein the closure structure is adapted to operably urge the rodagainst the dome upon the closure structure being positioned in thehead.
 21. A polyaxial bone screw assembly comprising: a) an elongateshank having a body for fixation in a bone and a capture structureextending from the body, the capture structure having a top and aradially outer surface with a first helically wound guide andadvancement structure; and b) a head having a base and a top portion,the head top portion having an open channel, the base having a loweropening with a second helically wound guide and advancement structuretherein that is sized and shaped to rotatingly mate with the first guideand advancement structure, the base further having a cavity openlyjoined to the opening, the channel communicating with the cavity, thecavity communicating with an exterior of the base through the opening,during assembly the capture structure being positionable in the cavityupon operable rotation of the first guide and advancement structure withthe second guide and advancement structure until a disengagement betweenthe structures occurs such that the capture structure is substantiallyin the cavity and the shank body is exterior of the cavity, the firstand second guide and advancement structures configured for frictionalengagement when the capture structure is urged toward the openingwithout rotation while the capture structure is within the head.
 22. Theassembly of claim 21 wherein the first and second guide and advancementstructures are first and second threads.
 23. The assembly of claim 22wherein the first thread is an inverted buttress thread.
 24. Theassembly of claim 21 wherein the capture structure has a lowersubstantially spherical surface, a portion of the spherical surfaceconfigured for frictional engagement with the second guide andadvancement structure.
 25. The assembly of claim 21 wherein the capturestructure has a tool engagement formation disposed thereon adapted fornon-slip engagement by a tool for driving the shank body into bone. 26.The assembly of claim 25 wherein the capture structure tool engagementformation is an axial projection having a hexagonal profile.
 27. Theassembly of claim 26 wherein the tool engagement formation is aprojection and the capture structure has a tool seating surface, theprojection and the tool seating surface partially defining a recessadapted for receiving a driving tool engaged with the tool engagementprojection and wherein the driving tool is adapted to be in contact withthe tool seating surface when driving the shank body into bone.
 28. Theassembly of claim 27 wherein the capture structure has at least oneengagement wall disposed perpendicular to the tool seating surface andspaced from the projection, the engagement wall at least partiallydefining the recess for receiving the driving tool and wherein thedriving tool is adapted to be in contact with the tool seating surface,the projection and the engagement wall when driving the shank body intobone.
 29. The assembly of claim 21 wherein the shank is cannulated fromthe top to a bottom thereof.
 30. The assembly of claim 21 furthercomprising a closure structure insertable into the head, the closurestructure being adapted to operably urge the shank in a direction tofrictionally lock the capture structure to the head, thereby locking theshank body in a selected angle with respect to the head.
 31. Theassembly of claim 30 wherein: (a) the head has upstanding spaced armsdefining the open channel, the arms having a third guide and advancementstructure on an inside surface thereof; and (b) the closure structurebeing sized and shaped to be positionable between the arms for closingthe channel, the closure structure having a fourth guide and advancementstructure for rotatably mating with the third guide-and advancementstructure.
 32. The assembly of claim 31 wherein the capture structuretop has a dome sized and shaped to extend into the channel so as to beadapted for engagement with a rod when received in the head and whereinthe closure structure is adapted to operably urge the rod against thedome upon the closure structure being positioned in the head.
 33. Apolyaxial bone screw assembly comprising: a) an elongate shank having abody for fixation to a bone and a capture structure extending from andintegral with the body, the capture structure having a top, a radiallyoutward facing surface with a first helically wound guide andadvancement structure, and a tool engagement formation disposed adjacentthe top, the tool engagement formation having a projection and arecessed tool seating surface having a bottom and an outer wall, boththe bottom and outer wall sized and shaped for receiving andfrictionally engaging a driving tool engaged with the tool engagementprojection when driving the shank body into bone; and b) a head having abase with an inner surface defining a cavity, a portion of the innersurface having a second helically wound guide and advancement structuresized and shaped to rotatingly mate with the first guide and advancementstructure, the base cavity communicating with an exterior of the basethrough an opening associated with the second guide and advancementstructure, the capture structure being pivotally disposed and capturedin the cavity upon operable rotation of the first guide and advancementstructure with the second guide and advancement structure until thefirst guide and advancement structure is disengaged from the secondguide and advancement structure.
 34. The assembly of claim 33 whereinthe first and second guide and advancement structures are first andsecond threads.
 35. The assembly of claim 34 wherein the first thread isan inverted buttress thread.
 36. The assembly of claim 33 wherein theshank is cannulated from the top to a bottom thereof.
 37. The assemblyof claim 33 further comprising a closure structure insertable into anupper opening of the head, the closure structure being adapted foroperably urging the shank in a direction to frictionally lock thecapture structure to the head, thereby locking the shank body in aselected angle with respect to the head.
 38. The assembly of claim 37wherein: (a) the head has upstanding spaced arms defining an openchannel, the arms having a third guide and advancement structure on aninside surface thereof, the channel communicating with the cavity; and(b) the closure structure is sized and shaped to be positionable betweenthe arms for closing the channel, the closure structure having a fourthguide and advancement structure for rotatably mating with the thirdguide and advancement structure.
 39. The assembly of claim 38 whereinthe capture structure top has a dome sized and shaped to extend into thechannel when the capture structure is urged against the opening, thedome being adapted for engagement with a rod when received in the headand wherein the closure structure is adapted to operably urge the rodagainst the dome upon the closure structure being positioned in thehead.
 40. A polyaxial bone screw assembly and bone screw implantationmethod comprising: a) attaching a bone screw shank to a head by mating afirst helically wound guide and advancement structure disposed on anupper portion of the bone screw shank with a second helically woundguide and advancement structure of the head to traverse the shankthrough an opening in the head and into a cavity in the head until thefirst guide and advancement structure is disengaged from the secondguide and advancement structure with the upper portion captured in thecavity; and b) driving the shank body into bone by rotating the shankbody with a tool engaged with a tool engagement formation disposed onthe upper portion.
 41. The method of claim 40 wherein step b) isperformed after step a).
 42. The method of claim 40 wherein step b) isperformed before step a).